‘If anyone has a problem with it I have not nice words to say to them.You drink breast milk from animals; it’s not that weird to drink breast milk from another human,’ she says, adding: ‘I don’t care what anyone else has to say – we do what’s best for our families.’

She adds that it’s nice to know her daughter still has that comfort when she has to be away from her and that she isn’t concerned her daughter will develop an overly-close bond with her friend.

So far, the friends haven’t faced any criticism for their wet nursing arrangement, but they realise that some will regard it as controversial.

Chrystal with her son Aiden and her daughter Ariana, and Stefani with her sons Gage and Noah (Picture: Ruaridh Connellan/Barcroft USA)

Viruses and infections including hepatitis and HIV can be spread through breast milk, but Stefani and Chrystal say they trust each other and what they put in their bodies.

‘My partner was sceptical because she’s unsure about what Stefani is putting into her body,’ says Chrystal. ‘Not Stefani specifically, but just anyone wet nursing her.

‘But we know Stefani has a pretty well balanced diet and she doesn’t smoke or drink.

‘We were both pregnant at the same time and we were tested for infections and both of us were clear.

‘The fact that Ariana is getting milk from the tap, I mean that’s exactly how we should drink it,’ she adds.

While the practice of using wet nurses is ancient and common to many cultures, it has been seen as taboo in modern western society. However, it is believed to be gaining popularity again.

A survey by NetMums earlier this year found that one in 25 British mums are already wet nursing a friend or relative’s baby. The survey also revealed two in five mums would breastfeed a stranger’s child.